Why choose between Windows 7 and Snow Leopard when you can have both? A Mac with virtualization software is a great platform for running Mac OS X, Windows, Linux, or other Intel-based operating systems, all at the same time. There's also Mac OS X's native Boot Camp, but it only supports Windows and doesn't give you access to Mac OS X without rebooting.

The latest versions of the Mac virtualization products from Parallels, VMware, and Sun offer significant improvements over previous versions, and all are worth the upgrade. They're faster with better 3-D graphics, are better integrated with Mac OS X, and in two cases, are optimized for running all the features of Windows 7.

Overall, Parallels Desktop 5 for Mac is the top virtualizer for Mac OS X. VMware Fusion 3 is a close second, with Sun's VirtualBox 3.1 running a distant third. VirtualBox has a few unique features and is free, but doesn't support many Mac OS X features. Parallels Desktop 5 and VMware Fusion 3 also automate the installation of guest operating systems and support multiple monitors. Furthermore, these new versions add support for Windows 7 Aero features, such as Aero Peek and Aero Glass. VirtualBox doesn't do any of these.

Parallels Desktop 5 provides the best overall performance. Not that VMware Fusion 3 is slow, but it can stumble with graphics-heavy tasks and uses more of the Mac's processor, leaving less CPU bandwidth for Mac applications.

On interface and Mac OS X integration issues, the merits of Parallels and VMware are more subjective. Both do a good job of hiding the Windows desktop and integrating Windows applications in the Dock, Expose, and Spaces. Both are far more advanced than VirtualBox's Seamless mode.

If you want to run Mac OS X Server in a virtual machine, however, VMware Fusion 3 is the clear choice, providing the most trouble-free and solid experience. Parallels can have trouble installing or importing Apple's server in a virtual machine, and VirtualBox just doesn't support it.

Where Parallels beats VMware Parallels Desktop 5 and VMware Fusion 3 both have new support for DirectX 9.0c Shader Model 3, OpenGL 2.1, and the Windows WDDM driver. These graphic hardware acceleration technologies enable support for Aero in Windows 7 and Vista, as well as 3-D gaming. Parallels Desktop 5 goes one step further and supports OpenGL 2.1 in Linux guest operating systems, enabling the Compiz interface to run in a virtual machine.

Despite similar graphics specs in Windows, Parallels is a little faster and can be more responsive. The differences are most noticeable when running Windows 7 Aero. VMware Fusion took 33 percent longer to start Windows and four times longer to restore a saved state from a suspended state. Parallels Desktop is also faster when switching to full-screen mode and when launching Windows applications, particularly when in a mode that hides the Windows desktop. This was on a 2.8GHz MacBook Pro with 4GB of RAM.

The CPU utilization, as measured by Activity Monitor, was typically lower with Parallels when Windows 7 Aero was running. When starting Windows 7 inVMware Fusion, CPU utilization reached above 150 percent, which temporarily stopped the Mac when other Mac applications were open. When starting Windows 7 in Parallels Desktop, CPU utilization spiked for a second to 130 percent, but mostly stayed below 50 percent.

The most hardware-intensive mode is one that hides the Windows desktop and taskbar, and displays only the windows running applications and Windows Explorer. Parallels has two such modes: Coherence and the new Crystal. VMware calls its integrated mode Unity. All three virtualization products can also run a guest operating system in its own window or in full-screen mode.

Because the Windows taskbar is hidden in Coherence and Unity modes, Parallels and VMware will minimize free-floating Windows windows to the Dock, just like Mac apps. Open Windows applications get their own Dock icons as well. Both virtualization programs will also place Windows apps in the Mac's Application Switcher (Command-Tab or four-finger horizontal swipe on trackpads) and display Windows application windows in Mac OS X's Expose.

Parallels Desktop goes a little further with this Mac integration, applying these features when the Windows desktop is displayed (single-window mode). Fusion supports Dock icons for Windows apps only in Unity mode.

Parallels is also more Mac-like in enabling you to launch Windows applications from Mac OS X, even when Parallels isn't running. It puts a Windows application folder in the Dock, just as Snow Leopard has a folder for Mac apps. Add a second Windows virtual machine, and a second folder is tacked on. Fusion appends a Start menu to the right side of the Mac's top-screen menu bar. It works, but I prefer the more standard Dock approach.

Additionally, when a virtual machine is running in Coherence mode, the Parallels Dock icon doubles as a Windows start menu. If you can't keep track of which mode supports what, a screen appears when you switch modes, telling you where you can find these items. (The new Crystal mode is a minimalist version of Coherence. It hides the Parallels Desktop menu and Dock icon, leaving only a small Parallels icon in the Mac menu bar at the top of the screen.)

Another nice touch: When you switch between modes, a transparent dialog appears, telling you what mode you're entering and showing you where you'll find various interface elements. One of my favorite Parallels features is complete support of track pad and Magic Mouse gestures in Windows applications. For instance, the multifinger horizontal swipe works in Internet Explorer to go forward or backward.

Where VMware beats Parallels That said, VMware Fusion 3 is a solid virtualization environment that some people may prefer. For instance, I had better experience playing Web-based video in Internet Explorer with VMware Fusion than with Parallels Desktop. VMware was able to play streaming video at Microsoft.com without adjusting anything. Parallels played only downloaded videos.

VMware Fusion 3 also has some useful interface features not found in Parallels Desktop. In full-screen mode, Fusion presents a small bar when you mouse over the center of the top edge of the screen. This bar presents options that let you do anything Fusion allows, including switch to single-window or Unity mode, suspend the virtual machine, take a snapshot, and a number of other tasks. In Parallels Desktop, you have only one choice when in full-screen mode: to exit the mode into single-screen mode.

Fusion does a better job with its virtual machine library, which is a window listing all the installed virtual machines. Fusion's library lets you turn on and suspend multiple virtual machines and access their settings, all in one spot. In Parallels Desktop, clicking on a virtual machine in the list opens its window, covering the library window.

VMware is less cluttered when switching between Unity, single-window, and full-screen modes. Parallels Desktop 5 uses some standard Mac OS X effects in nonstandard ways, which is at first confusing, then annoying, as well as completely gratuitous. For instance, when going in and out of Crystal or Coherence mode, Parallels uses the same screen-sliding effect that Spaces uses to switch between spaces -- except Parallels isn't switching you to another Space. Parallels also uses the rotating cube effect to go to full-screen mode; this is the effect Mac OS X employs to switch users, but in this case, users aren't being switched. Fortunately, you can turn these off in Parallels Desktop's Preferences.

VMware Fusion is the best choice for running Mac OS X Server in a virtual machine -- a very handy thing to do for testing a server before rolling it out. Installing Mac OS X Server versions 10.5 and 10.6 was trouble free; the server operated just fine running services on the network, and it didn't require futzing around with virtualization settings.

In addition, Mac OS X Server ran faster in VMware Fusion than in Parallels Desktop. Parallels was downright finicky with Mac OS X Server. I only got it to work after installation failed several times. Parallels was also unable to import a Snow Leopard Server virtual machine from VMware Fusion, although it does support moving Windows and Linux virtual machines from Fusion.

Where VirtualBox falls short VirtualBox provides good basic virtualization, without the ease-of-use and polished user interface of VMware and Parallels. But you can see the differences between VirtualBox and the others as soon as you install a guest operating system. Parallels Desktop and VMware Fusion automate the installation of Windows, filling in user information and advancing the process. With VirtualBox, it's up to you to click through the screens and select the information.

Importing existing virtual machines from other sources isn't straightforward. For instance, if you went directly to the Import command, you'd be headed in the wrong direction. And while VMware and Parallels support the importing of each other's virtual machine formats, VirtualBox doesn't support importing the current virtual machine formats of either Parallels Desktop 5 or VMware Fusion 3. (VirtualBox supports Desktop 2 and Fusion 2.) I was able to import an Ubuntu Linux virtual machine downloaded from the VMware Website, but only after experiencing problems with a perpetual spinning beach ball that left force-quit the only option.

Once you're running a virtual machine, you won't find the interface niceties of Fusion and Parallels Desktop. For instance, getting out of full-screen mode is done strictly by key command. VirtualBox's answer to Coherence and Unity is called Seamless, but it's not as integrated into Mac OS X as VMware Fusion and Parallels Desktop. Seamless simply hides the guest operating system's desktop, while keeping the Windows (or Linux) task bar at bottom, just above the Dock. Sometimes it appeared behind the Dock, which isn't all that useful. Windows applications don't get individual icons in the Dock or in the Application Switcher, and they don't integrate with Expose.

Graphic performance in Seamless mode can be a bit unresponsive, and it was a bit jerky when dragging windows around. And this is without Windows' hardware-hungry Aero feature, which VirtualBox doesn't support.

For Mac Pro users with eight processor cores, VirtualBox 3.1 enables users to assign up to 32 virtual processors to a single virtual machine, compared to 4 for VMware Fusion 3 and 2 for Parallels Desktop. Assigning multiple virtual CPUs makes a virtual machine run faster. But because you can't assign more virtual CPUs than the number of real processing cores, most Mac users can't take advantage of this feature. For all Macs except the Mac Pro and the Xserve, 2 virtual CPUs is the most you can assign to a single virtual machine in any of these virtualization products. With VirtualBox, when I tried to assign more than one virtual processor to a virtual machine on a dual-core MacBook Pro, a message stated that the setting is "non-optimal."

With Version 3.1, Sun added a unique feature called Teleportation. This is the ability to copy a virtual machine from one computer to another while the virtual machine is running, regardless of whether the host is a Mac, a Windows PC, or a Linux PC. It's an interesting technology that could be useful in enterprise situations, but the average Mac user is not likely to ever work with it.

VirtualBox's most compelling feature is its price: free. If you only need to run Windows XP or don't care about 3-D graphics, and you don't mind not having the Mac OS X integration features, VirtualBox 3.1 will work. But with list prices of $80, Parallels Desktop 5 and VMware Fusion 3 don't exactly break the bank.